The present invention relates to an implant for insertion between the vertebrae of the spinal column, whereby the cover surfaces of the substantially rectangular- or trapezoidal-section implant facing the adjacent vertebrae are porous and/or are provided with a profiled structure.
Implants of this type are used to form supports for vertebral fusions, whereby, after at least a partial removal of an intervertebral disc and preparation of the roof plates of the vertebrae, the implant that is to be inserted between adjacent vertebrae is introduced into the intervertebral interstice, and it is ensured that the intervertebral space is kept clear. By forming the cover surfaces facing the adjacent vertebrae with a porous surface or providing them with a profiled structure, the arrangement will become firmly anchored after the implant has been inserted due to the growth of the bones of the adjacent vertebrae onto the cover surfaces of the implant. In addition, or, as an alternative, at least one break-through or recess in the cover surface of such an implant may be filled with bone mass before being inserted so that the bone mass accommodated in the implant will be urged to knit with the material of the immediately adjacent vertebrae after the implant has been inserted. Moreover, a support system extending over a plurality of vertebrae may be provided externally of the spinal column near the implant for lending additional support, said system being attachable to the individual vertebrae.
Implants of the type mentioned hereinabove can be derived from U.S. Pat. No. 4,834,757, U.S. Pat. No. 5,192,327 or WO 90/00037 for example. In the construction according to U.S. Pat. No. 4,834,757, implants of substantially rectangular cross-section are made use of, whereby two implants are inserted posteriorly between respective pairs of adjacent vertebrae in each of the two side regions of the vertebrae. The disadvantage of this known construction is that, in addition to an at least partial removal of the rear bony and ligamentary elements which is necessary to allow insertion of the implant, it is imperative that two separate implants be used, this thereby not only resulting in increased expenditure for the operation but also requiring that they be mutually correctly positioned, something which it is not easy to achieve.
In the construction according to WO 97/00037, an implant is inserted whose outer dimensions substantially correspond to the total dimensions of two adjacent vertebrae. A particular disadvantage with implants of this type is that in practice they can only be inserted anteriorly, whereby however, in addition to the complicated access via the abdominal cavity with consequent possible injury to the major abdominal vessels and the nerve plexus, the frontal longitudinal ligament has to be cut-through so that the natural supporting function exercised by this longitudinal ligament is no longer available after the implant has been inserted. Moreover, a further operation then has to be effected posteriorly for the insertion of an additional support structure that thereby becomes necessary.
In addition to these embodiments of implants for insertion between vertebrae of the type described above, other implants having substantially cylindrical contours are known, for which reference may be made to EP-A 0 369 603 or DE-C 36 37 314 for example. Such types of cylindrical or tube-shaped implants may be additionally provided with a thread-like outer contour in order to enable them to be screwed between the vertebrae by means of a self-threading action. The particular disadvantage of tube-shaped implants of this type, whereby two implants must again be used between each two adjacent vertebrae in normal circumstances, is that the implants will not have a defined substantially flat support area on the vertebrae thus giving rise to the fear that difficulties will possibly be encountered in regard to the growth of the incorporated bony material. Another disadvantage is that injury to soft bony parts with consequential breakage or subsidence of the bone may be induced by the threading or screwing actions.
Based upon the above-mentioned state of the art, the aim of the present invention is then to provide an implant for insertion between the vertebrae of the spinal column, whereby an adequate and reliable supporting function can be achieved using merely one implant between two vertebrae and wherein the implant in accordance with the invention should be insertable posteriorly, especially laterally, so as, firstly, to simplify the operation and enable the implant to be inserted together with an additional support system if required and secondly, to avoid the necessity of damaging or destroying naturally provided support elements such as the frontal longitudinal ligament for example and/or to avoid having to remove additional ligamentary and bony material, especially in the region of the vertebral or spinous process, in order to allow the implant to be inserted.
Based upon an implant of the type mentioned hereinabove, the above object is achieved by the implant in accordance with invention, in essence, in that the breadth of the implant is such that it covers at most the rearward half of the vertebra facing the spinous process, and preferably at most the rearward third thereof, and the length of the implant is essentially such that it overlaps the rearward region of the vertebra, and in that the boundary face of the implant facing the interior of the vertebra and extending towards the adjacent vertebrae is curved convexly. Due to the fact that the implant in accordance with the invention basically covers only the rearward portion of the adjacent vertebrae when it is in the installed state, the requisite reach can be found with an appropriately miniature implant, whereby the largest forces and loads are effective in the rear portion of the vertebrae facing the vertebral or spinous process and can thus be reliably accommodated in a corresponding manner by the implant in accordance with the invention, and also the bone strength is at its greatest in said rear portion. In addition to a miniature, one-piece construction of the implant as proposed in accordance with the invention, it is also possible to introduce the implant laterally between and passing-by the protruding nerve roots and the spinal cord into the intervertebral space from behind, especially by virtue of the further matter envisaged in accordance with the invention, namely, the convexly curved boundary face facing the interior of the vertebra when in the installed state, so that, in the case of a posterior operational technique of this type, not only will the frontal longitudinal ligament remain completely undamaged, but one can also dispense with the additional removal of ligamentary and bony material such as was required for implants in accordance with the state of the art. By virtue of the curved boundary face, not only is it possible to ensure a simple sideways introduction of the implant, but exact positioning of the implant can also be achieved so as to cater for the portion of the vertebrae facing the spinous process being covered in the inserted state and thereby providing proper support.
In order to simplify still further the lateral introduction of the implant, provision is preferably made for the boundary face of the implant facing the spinous process to be curved concavely so that overall, when seen from a top view, there will be provided an essentially bow or crescent shaped implant which is correspondingly easily and reliably introducible from the side into the intervertebral space from the rear with the very smallest amount of space being required.
Moreover, for the purposes of adequately supporting adjacent vertebrae by covering an appropriate portion thereof, it is proposed that the radius of curvature of the boundary face of the implant facing the interior of the vertebra amount to 30 to 90%, and especially to 40 to 80%, of the spacing between the frontal edge of the vertebra and the peak of the spinous process, as corresponds to a further preferred embodiment of the implant in accordance with the invention.
For the purposes of matching the outer contours of the vertebrae in the portion facing the spinous process and also for maintaining sufficient spacing between the implant and the vertebral canal, it is proposed in another preferred embodiment that the radius of curvature of the boundary face of the implant facing the spinous process amount to 10 to 80%, and especially to 20 to 60%, of the spacing between the frontal edge of the vertebra and the peak of the spinous process. Thus, in the case of the implant in accordance with the invention, adaptation to vertebrae of various sizes or to vertebrae located at differing parts of the spinal column can be obtained by appropriate selection of the radius of curvature of the boundary faces.
As was mentioned hereinabove, the implant may be filled with bone mass prior to insertion in order to enable it to knit with the surrounding bone material of the vertebra so as to achieve the desired level of support in the subsequent part of the process. However, in order to prevent a build up of bone mass from the implant covering the rearward section of the vertebrae towards the spinous process, and especially towards the vertebral canal, additional provision is preferably made for the boundary face facing the spinous process to be substantially free of break-throughs or recesses. This boundary face thus makes a defined surface of the implant available from which there will be no fear of an issue of bone mass from the interior of the implant.
Since, when in its installed state, the implant in accordance with the invention merely covers a portion of the surface of the vertebrae, the intervertebral space located outside this portion may, in accordance with the invention, also be filled with bone material before the insertion of the implant and after the removal of the intervertebral disc thereby producing a substantially fully filled intervertebral space after the implant has been inserted. In this context and for the purposes of connecting the bone mass outside and within the implant in the intervertebral space, it is preferably proposed that the boundary face facing the interior of the vertebra comprise at least one recess or depression or break-through. Due to this at least one recess, depression or break-through in the boundary face facing the interior of the vertebrae, knitting or growth of the bone mass in the intervertebral space can thus be achieved, whereby this knitting of the implant with the surrounding bone material can easily be checked and confirmed by means of an X-ray examination for example. Furthermore, due to the formation of a depression or recess, a central, thinner region of the implant can be obtained so that it can be introduced more easily into the intervertebral space when the spatial conditions are very tight thereby encouraging the process of knitting with the surrounding bone material. Furthermore, an implant having a recess or depression of this type can be fed past the nerve root without displacement thereof during the insertion process. Moreover, a bone graft inserted in the intervertebral space not covered by the implant and having a bow-shaped or convex surface or outer contour can be placed in contact with such a depression or recess over a larger peripheral area thereby encouraging the subsequent knitting process.
In order to avoid injuring the region of the spinal column surrounding the implant and in order to ensure a substantially form-fitting seal between the implant and the outer contours of the vertebrae after the installation of the implant, provision is made for the outer faces of the implant facing the laminae to be rounded, and, in particular, to substantially follow the outer contour of the vertebra in the vicinity of the laminae, as corresponds to another preferred embodiment of the implant in accordance with the invention.
For the purposes of easily gripping the implant by appropriate instruments during the insertion process or when removing or replacing it, further provision is preferably made for a respective opening to be provided in the outer faces, said opening being provided, in particular, with a thread and being closable if required.
In addition to the provision of openings, especially those with a thread, for gripping or positioning the implant in accordance with the invention, provision is made in accordance with another preferred embodiment for at least one opening or boring extending, in particular, at an angle to the longitudinal axis of the implant to be provided, said opening commencing from at least one outer face and/or from the boundary face opposite the convexly curved boundary face and ending in the convexly curved boundary face facing the interior of the vertebra. By virtue of such an opening or boring extending, in particular, at an angle to the longitudinal axis of the implant and ending in the convexly curved boundary face facing the interior of the vertebra, it is possible, after the insertion of the implant, to subsequently compress the bone material located in the region within the vertebra by using an appropriate rod or tube-shaped tool or instrument, or, to introduce additional bone material through an opening of this type so as to ensure a corresponding firm retention of the implant and to assist in subsequent knitting of the implant to the surrounding bone material. Provision may hereby be made either for small pieces of bone material to be arranged in the region of the intervertebral space not covered by the implant, or, for a bone graft from the iliac crest for example to be inserted so that, by virtue of the opening extending, in particular, at an angle to the longitudinal axis of the implant, it becomes possible for this bone graft to be subsequently appropriately positioned or for any possibly remaining free space to be subsequently filled with additional bone material or for proper compression thereof to be effected. Hereby, provision may further be made for the opening ending in the region of the outer surfaces and having a thread for co-operating with an instrument to be extended directly through the opening ending in the region of the convexly curved boundary face so that such a continuous opening or such a canal can be utilised firstly for inserting the implant by using an appropriate tool, and thereafter for stopping-up or filling the intervertebral space using an instrument or tool having a diameter less than that of the threaded portion.
In order to be able to traverse a further region of the intervertebral space through such an opening extending, in particular, at an angle to the longitudinal axis of the implant, provision is made in accordance with a further preferred embodiment for the opening or boring to be formed such that it widens out conically towards the convexly curved boundary face. A tool of appropriately small dimensions can thus be introduced into the intervertebral space at differing angles of inclination by virtue of such a conically widening opening.
In order to enable adaptation to the anatomically ordained curvature of the spinal column and to retain the bio-mechanical functioning of the spinal column, provision is made in another preferred arrangement for the cover surfaces of the implant resting on the adjacent vertebrae to include an acute angle therebetween, whereby the particularly preferred proposal is made that the angle included between the cover surfaces amount to at most 25xc2x0, and especially to at most 15xc2x0.
For the purposes of supporting the connection of the implant to the surrounding material of the vertebrae and to ensure firm anchorage of the implant, the boundary faces of the implant extending towards the adjacent vertebrae are made porous and/or are provided with a profiled structure, in particular, by means of a plurality of elevations and depressions, such as corresponds to another preferred embodiment of the invention. In accordance with an especially preferred embodiment, provision may be made in this context for the porous surface to be formed by sandblasting or vacuum deposition.
In addition to the hollow spaces or breakethroughs required for the bone mass filling process, it is necessary to provide an adequate support function and a firm anchorage for the implant which is comparatively long since it covers substantially the whole outer spacing of the laminae, so that, in accordance with the invention, provision is preferably also made for the implant to be substantially hollow at least in the direction of the adjacent vertebrae between the boundary faces facing the interior of the vertebra and the spinous process, whereby, in addition to the outer surfaces connecting the boundary faces, there is provided at least one web-like connection extending towards the outer surfaces between the boundary faces. The strength of the implant is corresponding increased by virtue of at least one such web-like connection, whereby however, an adequately large amount of free space still remains in the implant for filling it with bone mass or to allow subsequent growth.
Thus, the implant in accordance with the invention can be made to be substantially solid or holohedral and be substantially free of continuous break-throughs or recesses extending towards the adjacent vertebrae, thereby resulting overall in the provision of a large surface area for application to the adjacent vertebrae and hence better support therefor and an overall increase in the strength of the implant in accordance with the invention. In the case of a construction of this type, rooting or growth of the surrounding bone material, essentially on the cover surfaces and the boundary faces extending towards the adjacent vertebrae, is thereby obtained with consequentially secure anchorage of the implant in the intervertebral space. The profiled structures provided in the region of the cover surfaces and the boundary faces extending towards the adjacent vertebrae and/or an at least partially porous surface on these boundary faces serve to assist such rooting or outward growth. Due to the provision of at least one break-through extending towards the adjacent vertebrae so that the interior of the implant in accordance with the invention is at least partially hollow, the bone material disposed in the hollow cavity following the insertion of bone material into this hollow region can knit with the adjacent bone surfaces, or, bone material can grow directly through hollow cavities of this type.